Infectious diseases that affect dogs, cats and other animals having close interactions with humans are important not only from a veterinary standpoint, but also because of the risk to public health. An infectious disease is caused by the presence of organisms such as viruses, bacteria, fungi, or parasites (either animalian or protozoan). Most of these diseases are spread directly from animal to animal, while others require a vector such as a tick or mosquito. Certain infectious diseases are a concern from a public health standpoint because they are zoonoses (transmittable to humans).
Heartworm is a dog parasitoid. It is hard to eliminate and can be fatal; prevention, however, is easily achieved using medication. As the name suggests, an infected mosquito injects a larva into the dog's skin, where it migrates to the circulatory system and takes up residence in the pulmonary arteries and heart, growing and reproducing to an alarming degree. The effects on the dog are quite predictable, cardiac failure over a year or two, leading to death. Treatment of an infected dog is difficult, involving an attempt to poison the healthy worm with arsenic compounds without killing the weakened dog, and frequently does not succeed. Prevention is much the better course, via heartworm prophylactics which contain a compound which kills the larvae immediately upon infection without harming the dog. Often they are available combined with other parasite preventives. The definitive host for heartworm is dog but it can also infect cats, wolves, coyotes, foxes and other animals, such as ferrets, sea lions and even, under very rare circumstances, humans.
There are several species of Ehrlichia, but the one that most commonly affects dogs and causes the most severe clinical signs is E. canis. This species infects monocytes in the peripheral blood. Two conserved major immunoreactive antigens, gp36 and gp19, are the first proteins to elicit an E. canis-specific antibody response, while gp200 and p28 elicit strong antibody responses later in the acute phase of the infection. Recombinant polypeptides gp36, gp19, and gp200 (N and C termini) exhibited 100% sensitivity and specificity for immunodiagnosis by the recombinant glycoprotein enzyme-linked immunosorbent assay (ELISA) compared with the results obtained by an indirect fluorescent-antibody assay (IFA) for the detection of antibodies in dogs that were naturally infected with E. canis. Cárdenas et al. (2007) Clin. Vacc. Immunol. 14:123-128.
A. phagocytophilum is a Gram negative, obligate bacterium of neutrophils. It is also known as the human granulocytic ehrlichiosis (HGE) agent, Ehrlichia equi, and Ehrlichia phagocytophila, and is the causative agent of human granulocytic anaplasmosis, tick-borne fever of ruminants, and equine and canine granulocytic anaplasmosis. See la Fuente et al. (2005) J. Clin. Microbiol. 43:1309-1317. A. phagocytophilum binds to fucosylated and sialylated scaffold proteins on neutrophil and granulocyte surfaces. A type IV secretion apparatus is known to help in the transfer of molecules between the bacterium and the host. The most studied ligand is PSGL-1 (CD162). The bacterium adheres to PSGL-1 (CD162) through 44-kDa major surface protein-2 (Msp2 or P44). After the bacteria enters the cell, the endosome stops maturation and does not accumulate markers of late endosomes or phagolysosomes. Because of this the vacuole does not become acidified or fused to lysosomes. A. phagocytophilum then divides until cell lysis or when the bacteria leaves to infect other cells. See Dumler et al. (2005) Emerging Infec. Dis. 11.
B. burgdorferi is a species of Gram negative bacteria predominant in North America, but also exists in Europe, and is the agent of Lyme disease. Lyme disease clinical features include the characteristic bull's eye rash and erythema chronicum migrans (a rash which spreads peripherally and spares the central part), as well as myocarditis, cardiomyopathy, arrythmias, arthritis, arthralgia, meningitis, neuropathies and facial nerve palsy.
A variety of serologic tests, such as IFA staining methods, Western blot analysis, and ELISAs, have been used to verify past or current infections of B. burgdorferi and A. phagocytophilum infections. Although sensitivities and specificities of these assays were considered acceptable, there is potential for false positive reactions when whole-cell antigens are used because heatshock, flagellin, or other proteins of these pathogens may be shared with other bacteria. Recent advances in the production and use of purified recombinant antigens (i.e., fusion proteins) in ELISAs to detect antibodies in human, dog, horse, and bovine sera have improved laboratory analyses. See IJdo et al. (1999) J. Clin. Microbiol. 37:3540-3544; Magnarelli et al. (2001) Eur. J. Clin. Microbiol. & Infect. Dis. 20:482-485; Magnarelli et al. (2001) J. Med. Microbiol. 50:889-895; Magnarelli et al. (2001) Am. J. Vet. Res. 9:1365-1369; Magnarelli et al. (2002) J. Med. Microbiol. 51:326-331; Magnarelli et al. (2002) J. Med. Microbiol. 51:649-655. The B. burgdorferi OspA, OspC, OspF, p39, p41 and VLsE antigens and A. phagocytophilum p44 antigen have been all shown to have some, but not 100%, seropositivity. Magnarelli et al. (2004) J. Wildlife Dis. 40:249.